1. Field of the Invention
This invention pertains to magnetic field sensors generally, and particularly to methods for adjusting sensor linearity while simultaneously stabilizing permanent magnet output.
2. Description of the Related Art
There are a variety of known techniques for position sensing. Optical, electrical, electrostatic and magnetic fields are all used with apparatus to measure position. A few of the known apparatus are resistive contacting sensors, inductively coupled ratio detectors, variable reluctance devices, capacitively coupled ratio detectors, optical detectors using the Faraday effect, photo-activated ratio detectors, radio wave directional comparators, and electrostatic ratio detectors. There are many other known detectors, too numerous to mention herein.
These sensors each offer much value for one or more applications, but none meet all application requirements for all sensing applications. The limitations may be due to cost, sensitivity to particular energies and fields, resistance to contamination and environment, stability, ruggedness, linearity, precision, or other similar factors.
Sensing applications can be very demanding. The sensor may be immersed in or sprayed by contaminants. This may occur at all temperature extremes with the sensor operational in heated compartments or in near absolute zero scientific exploration.
The combination of temperature extremes and contamination causes industry to explore very rugged and durable components. One particular group of sensors, those which utilize magnetic energy, are rapidly being accepted into demanding applications. This is because of the inherent insensitivity of the magnetic system to contamination, together with great durability characteristic of the components.
Applying magnetic sensing to ferromagnetic tone wheels has been a relatively easy task. The impulse provided by the tone wheel is readily detected through all conditions, with very simple electronic circuitry. Due to the digital nature of the output, no linearity is required of a tone wheel or similar digital proximity system.
Linear analog magnetic sensors, particularly those using Hall effect IC detectors, are also being pursued. Industry believes these sensors will offer advantages over more widely used existing technologies. Magnetic circuits offer admirable performance upon exposure to commonplace moisture and dirt contaminants. However, prior to the present invention, none of the magnetic sensors were able to offer the necessary combination of low cost, reliability, and precision output.
Magnetic sensors are not currently used in applications where there is a need for linearity and precise output. Deviations in linearity of less than one percent from sensor output compared to actual may have very adverse affect on perceived quality and even on related electronic control functions. While many sensors are specified for extremely tight tolerances, magnetic sensing has previously not met these requirements.
Typical magnetic circuits use one or a combination of magnets to generate a field across an air gap. The magnetic sensor, be this a Hall effect device or a magnetoresistive material or some other magnetic field sensor, is then inserted into the gap. The sensor is aligned centrally within the cross-section of the gap. Magnetic field lines are not constrained anywhere within the gap, but tend to be most dense and of consistent strength centrally within the gap. Various means may be provided to vary the strength of the field monitored by the sensor, ranging from shunting the magnetic field around the gap to changing the dimensions of the gap.
Regardless of the arrangement and method for changing the field about the sensor, the magnetic circuit must be precise in spite of fluctuating temperatures. In order to gain useful output, a permanent magnet must be completely saturated. Failure to do so will result in unpredictable performance. However, complete saturation leads to another problem referred to in the trade as irreversible loss. Temperature cycling, particularly to elevated temperatures, permanently decreases the magnetic output.
A magnet also undergoes aging processes not unlike those of other materials, including oxidation and other forms of corrosion. This is commonly referred to as structural loss. Structural and irreversible loss must be understood and dealt with in order to provide a reliable magnetic sensor with precision output.
Prior art magnetic sensors are illustrated, for example, by Tomczak et al in U.S. Pat. No. 4,570,118. Therein, a number of different embodiments are illustrated for forming the magnetic circuit of a Hall effect position sensor. The Tomczak et al disclosure teaches the use of a sintered samarium cobalt magnet material which is either flat, arcuate, and slightly off-axis, or in second and third embodiments, rectangular with shaped pole pieces. The last embodiment is most relevant to the present invention, where there are two shaped magnets of opposite polarity across an air gap of varying length.
No discussion is provided by Tomczak et al for how each magnet is magnetically coupled to the other, though from the disclosure it appears to be through the use of an air gap formed by a plastic molded carrier. Furthermore, no discussion is provided as to how this magnetic material is shaped and how the irreversible and structural losses will be managed. Sintered samarium cobalt is difficult to shape with any degree of precision, and the material is typically ground after sintering. The grinding process is difficult, expensive and imprecise. The device may be designed through shaping to be linear and precise at a given temperature and a given level of magnetic saturation, presumably fully saturated. However, such a device would not be capable of performing in a linear and precise manner, nor be reliable, through the temperature cycling realized in many environments. This is, as noted, because of the irreversible and structural losses.
Ratajski et al in U.S. Pat. No. 3,112,464, Wu in U.S. Pat. No. 5,159,268, and Allots in U.S. Pat. No. 5,164,668 illustrate several embodiments of brushless Hall effect potentiometers. However, as in Tomczak, there is no provision for temperature stabilization nor improving the linearity in a non-intrusive manner.